The known solvent-containing coating materials, more particularly those known as basecoat and clearcoat materials, and the single-coat or multicoat, color and/or effect paint systems produced using them feature good performance properties.
The continually growing technical and esthetic requirements of the market, especially the requirements of the car makers and their customers, however, are necessitating continual ongoing development of the technical and esthetic level hitherto achieved.
More particularly it is necessary to provide new coating compositions which have a lower VOC as compared with the prior art, so that fewer VOC emissions occur in the production of corresponding multicoat paint systems, and, consequently, the production process can be made more environmentally safe. A particular way in which the VOC can be lowered is by increasing the solids content of the coating materials in question.
Reducing VOC emissions can also be accomplished, however, by a reduction, for example, in the film thicknesses of coating compositions that are used for producing multicoat paint systems, and especially of solventborne, unpigmented coating compositions.
Nowadays, for example, high-solids clearcoat materials (clearcoat materials with a high solids content of above 50%) are applied generally in film thicknesses of around 45-55 μm, in order to obtain effective leveling and a very good overall appearance. Reducing the clearcoat film thickness to 35-40 μm can lead to a lowering of VOC emissions by about 5 g per square meter of coated surface.
Besides the aspect of environmental safeness, however, it is critical that at the same time other technical properties and also the aesthetic properties of coating materials and of multicoat paint systems produced from them are retained, if not, indeed, improved.
In particular, the advantages achieved by means of the known basecoat and clearcoat materials in the production of multicoat paint systems are not to be lost, but instead are to be retained at least to the same extent and to a greater extent.
Multicoat paint systems comprising basecoat and clearcoat are widespread in the automotive industry. They are used on account of their outstanding profiles of properties, such as scratch resistance, chemicals resistance and weather resistance, and also their high gloss.
The high-solids (low-VOC) clearcoat materials that are used in the production of such multicoat paint systems for reasons of environmental protection, and which nevertheless possess the high scratch resistance required by the customer, are based predominantly on carbamate-containing binder systems, which in combination with monomeric crosslinking resins such as hexa(methoxymethyl)melamine (HMMM) or melamines with mixed etherification, and with further polymerizable binders, are able to form an impervious network.
The basecoat materials that are used in producing multicoat paint systems generally comprise a polymerizable binder and a crosslinker. The polymerizable binder here frequently possesses hydroxy-functional groups on a polymeric scaffold. Crosslinkers used are monomeric crosslinking resins such as hexa(methoxymethyl)melamine (HMMM) or melamines with mixed etherification.
Multicoat paint systems are produced, for example, by applying a clearcoat material, after having carried out preliminary application of a pigmented basecoat material and after a brief flash-off time without a baking step (wet-on-wet process), and then baking basecoat and clearcoat materials together.
In the context of the wet-on-wet process as well, the reduction of VOC emissions when producing multicoat paint systems possesses a high relevance. As a result of the switch from medium-solids basecoat systems, with a solids content between 20-30%, to what are called high-solids basecoat systems, with a solids content greater than 35% in the application-ready state (spray viscosity), a significant VOC reduction in the processing operation is achieved. Moreover, increasing the solids content by about 5% to 10% in the context of industrial coating operations already implies a massive saving in organic solvents that are used absolutely and are therefore emitted to the environment in the course of processing. A further reduction in overall solvent emissions can be accomplished by reducing the clearcoat film thickness.
A particular problem with the reduction in the clearcoat film thickness is that the leveling properties and hence also the optical quality of the multicoat paint systems produced are significantly lowered.
It would therefore be advantageous to have a pigmented coating composition, more particularly a basecoat material, which allows good leveling properties and also a very good overall appearance to the multicoat paint systems produced, even with low film thicknesses of clearcoat materials applied to the pigmented paint, but which, at the same time, does not result in any deterioration in other processing properties, applications properties and, in particular, technological properties of the coating compositions employed and also of the multicoat paint systems produced. It would be advantageous, furthermore, if the basecoat material itself had a high solids content, so as to make the production of multicoat paint systems even safer from an environmental standpoint.